Though our knowledge about autism spectrum disorders (ASD) has grown considerably over the last two decades, we know little about the clinical phenotype in adulthood. Recent studies suggest that anywhere from 10-20% of those diagnosed with ASD obtain favorable outcomes in young adulthood. At the same time, a growing body of literature suggests that for many, outcomes worsen during the transition from school-based services to adulthood. The primary aim of this project is to accurately describe ASD in adulthood as well as the transition to adulthood and to identify mechanisms that contribute to positive and negative outcomes in individuals with ASD and their families. The transition to adulthood will be examined in a well-described longitudinal sample of children referred for possible ASD at age two, and a group of non-spectrum developmentally delayed controls most of whom have been followed for two decades. The current study will follow the probands as they transition to adulthood; families will receive annual assessment packets over the next 5 years and receive two face-to-face assessments 3 years apart. The longevity of this study and the families' continued participation will aid our recruitment efforts and should allow us to set up a framework for collection of biological samples from this extraordinary group of families which would then be made available to interested scientists. Findings will inform best practices for the care and support of adults with ASD and ultimately lead to improvements in the quality of life for individuals affected by ASD and their families.